|MUTATION: Permanent transmissible change in the genetic material.|
As we have learnt in previous lectures, genes are sections of chromosomes which are threads of protein in the nucleus of plant and animal cells. Genes themselves are units of atoms, which are the smallest particles of matter that can exist. It happens sometimes that, when genes are being duplicated, one of the atoms is copied incorrectly. This causes the ‘new’ gene to be different from the original gene and in effect a new allele has been produced. The altered gene is occupying the same place on the chromosome as the old gene. This process is called Mutation.
Most new genes produced by this process of mutation are recessive and are detrimental, or even lethal to the animal. However, being recessive their effect is masked by a normal, dominant gene, so that they do not actually harm the animal. This is not always the case; and if they are dominant, and their effect is very bad, the animal will die, so that the mutation will die with the animal and not be passed on to the next generation.
In some cases, these mutant genes cause, in a species, genetic variations which are desirable and allow that species to survive when it would have died without the variation. In this way, mutations play an important part in evolution, when animals adapt themselves (over thousands of years) to survive in a different environment. It has been estimated that one gamete in every 20 000 carried a new mutation.
There are some genes which cause the death of a foetus inside the womb, or soon after the animal has been born. These are called Lethal Genes.
Most lethal genes are either recessive or partially dominant and they must be presentin the cells as homozygous genes to have their full effect. If they are recessive, they can be hidden in the parents and only show themselves in the offspring. Inbreeding increases the possibility of a lethal gene working in an offspring.
Examples of conditions caused by genes, and which can be lethal or harmful to an animal are:
|Hairlessness||Lack of hair on the newly born animal. This is a recessive gene.|
|Hypoplasia||Undeveloped gonads (sex organs). A recessive gene.|
|Umbilical Hernia in Frieslands||The hernia appears in the calf between 8 and 20 days old and persists until it is about 7 months old. This is caused by a dominant gene and is limited to males.|
|Bulldog Calves in Ayrshires||Calves are born dead, with large deformed heads caused by a recessive gene.|
Two animals with the same genotype will grow differently if they are reared in different environments. Although the genetic material may provide the messages and direction for good growth, the actual development of the animal will depend on environmental factors such as the amount and quality of the food, temperature length of daylight, etc. The genetic potential of the animal may be very good, but that potential will be fulfilled only if the animal is given good conditions. This is particularly true with milk yield in dairy cows, because, although the cow may be well bred, with the genetic potential for high milk yields, unless it is fed, milked and managed properly, it will never produce its maximum milk yields. In the same way, a bull may produce very good calves on one farm with a high standard of management, but if he is sold and used on another farm, where the management is poor, he may produce very poor calves.
In any breeding programme, the environment, which really means the management, must be first class so that any improvement which appears in the stock can be attributed to, genetic improvement.
If two pure bred or closely inbred lines of plants or animals are crossed, the offspring of the first cross, the F1 generation, are often healthier, and more vigorous than either of the parents. The offspring of such a cross is called a hybrid, and the condition of the plant or animal is called Hybrid Vigour.If two hybrids of the F1 generation are then mated, their offspring called the F2 generation, no longer show this extra vigour, and can be worse off than either of the parents.
The reason for Hybrid Vigour is that when plants or animals are closely bred to get pure breeding, homozygous lines, dangerous or undesirable recessive genes show themselves in the plant or animal because they are no longer masked by the dominant gene. To illustrate this, let us go back to the polled bull. Suppose we have a polled bull, and we want to know if he is pure breeding for polled, i.e. is he homozygous or heterozygous for being polled. We can find out by mating him with a few cows with horns. If he is pure breeding, or homozygous, for polled, the following results will be observed:
Figure 1: Results from Breeding with a Polled Bull and a Horned Cow
The result in every case will be polled calves, although these calves will all be carrying a recessive gene for horns. However, we now know for certain that the bull is pure breeding and homozygous for polled.
Consider the bull that is polled but is not pure bred. He is heterozygous for polled and is carrying a recessive gene for horns. Mate this bull with a few cows with horns and the following results will be observed:
Figure 2: A Heterozygous Polled Bull and a Homozygous for HornedCow
Out of 4 mating sessions, the chances are that there will be 2 polled calves and 2 calves that have horns. If there is any calf that grows horns, we will know for certain that the bull is not pure bred, or homozygous, for polled. It must be carrying a hidden recessive gene for horns.
To return to the earlier point, hidden recessives will be shown up in any pure breeding programme, and some of these recessives will affect the performance of the animal. They can affect it by making it more susceptible to disease, having a weaker bone structure, be a poor digester of food, having poor fertility, etc. When two pure bred plants or animals containing these recessives are crossed, many of the undesirable genes from the parents are masked in the offspring, and cannot affect the animal, so the offspring can grow better, be healthier and be more productive than the parents. Furthermore, it is possible to produce a hybrid that has the best characteristics of both parent, together with an extra bonus of hybrid vigour, and this fact is widely used in agriculture. There are many examples of this, and some of the following facts can be observed:
- Hybrid chickens – these are usually crosses between a light breed and a heavy or medium breed, whereby the egg size and health of a heavy or medium breed is combined with the low food consumption of the light breed; the bonus is the extra number of eggs produced by the hybrids;
- Hybrid pigs – these are crosses between a heavy, prolific breed like the large white, and a leaner, lighter breed like the Landrace. The offspring from this cross are very good bacon pigs that grow well and grade for bacon; they are not too fat;
- Sheep – one of the best examples of a sheep hybrid is the Scotch Half-breed sheep in the U.K. This is a cross between a Border Leicester ram and a Cheviot Ewe. The Border Leicester is a large breed with a lot of poor quality wool, while the Cheviot is a small, hardy mountain breed but with a smaller fleece of good quality wool. The Scotch Half-breed combines the hardiness of the Cheviot with the meat of the Border Leicester and it is a medium sized sheep with a good fleece of quality wool.
- In Southern Africa, we find the Dorper sheep which is a cross between a Dorset Horn, a breed from the U.K. which produces two crops of lambs a year, and the Black Head Persian Sheep which is a hardy local breed, with very short wool and able to withstand drought conditions. The Dorper is a fertile sheep with short wool, able to thrive under local conditions and able to lamb twice a year; and
- Cattle – any cross between a beef bull, such as a Hereford or an Aberdeen Angus, and a dairy cow such as a Friesland or a Holstein, produces a good animal which grows well and is not too fat for the butcher. In other words, it produces an ideal meat animal.
Heritability is the extent to which a characteristic in a plant or animal is governed by genetic factors. A characteristic with a high heritability means that there is a very good chance of its being passed on genetically from the parents to the offspring, whereas the characteristics with a low heritability will be largely affected by the environment and management of the animal. Heritability estimates have been worked out in great detail for various characteristics of different animals, and Table 1 below gives you some of these. A Heritability Estimate of 20% is low, and of 50% is high, and anything over 60% is very high.
Looking at the table, you can see that in dairy cows, milk yield has a low heritability, and good milk yields depend mainly on the feeding and management of the cow. Butterfat % has a high heritability of 50 % so that the butterfat average of a herd can be improved fairly quickly by means of a good breeding programme, using high butterfat cows. In the same way, in pigs, the weaning weight of a litter has a low heritability because this depends on how much milk the sow produces for her little pigs, whereas back fat thickness has a high heritability. If a boar with a low back fat thickness is used in a herd, the back fat of his progeny will be low.
Table 1: Estimates of Heritability Value for Different Characteristics
|CLASS OF STOCK||CHARACTERISTICS||HERITABILITY|
|Dairy Cattle||Milk Yield Butterfat Yield Butterfat Percentage||20 – 25% 20% 50%|
|Beef Cattle||Birth Weight Live Weight Gains Feed Conversion Carcass Grade||30% 60% 45 – 60% 50 – 60%|
|Pigs||Weaning Weight of Litter Litter Size Food Conversion Daily Gain Carcass Length Back Fat Thickness Width of Body||12 – 18% 10 – 15% 22 – 27% 25% 50% 45 – 55% 47%|
|Sheep||Birth Weight Yearling Weight Yearling Weight of Clean Wool Fecundity (No. of Lambs Born per Ewe) Yearling Staple Length of Wool||30% 40 +% 33% 10 – 15% 36%|
To give you a better idea of how heritability estimates have been worked out in detail, Table 2 below gives the figures for one breed of pigs the Large White.
Table 2: Heritability Estimates for British Large White Bacon Pigs
|Back fat thickness – shoulder – mid-back – loin – over eye muscle||62% 73% 71% 65%|
|Eye muscle width||46%|
|– depth – area||48% 35%|
|Carcass conformation score||31%|
|Back rasher score||59%|
|Food conversion (dead weight)||58%|
In general, characteristics which can be seen by inspection of the animal and its appearance or conformation, have a high heritability. Characteristics which cannot be seen, milk yield, litter numbers, birth weights, etc., have a low heritability.